The project was being developed by German firm PNE Wind. The 73-turbine development is planned for a site in the North Sea, 84km north-west of the island of Borkum.
PNE Wind acquired the project in 2013 and will remain involved in it as a service provider.
The financial details of the transaction were not released, but PNE Wind said it was for a "lower double-digit million-euro amount". There will also be future milestone payments as the project progresses.
"This project was prepared by us, so that it is eligible to participate in the first ever tender in Germany for offshore wind projects this year. This was instrumental in getting the deal done," said PNE Wind CEO Markus Lesser.
Germany's offshore wind projects due for installation between 2021-2025 will be selected by two auctions, scheduled for 1 March 2017 and 1 March 2018.
The country's netwrok regulator assumes Germany's offshore wind capacity in 2025 will amount to 10.75GW, of which 8.93GW will be located in the North Sea and 1.82GW in the Baltic Sea.
Germany's Offshore Wind Act specifies that in 2021, only installations in the Baltic Sea are allowed, and not to exceed 500MW.
In 2022, the Act allows a total 500MW of offshore wind installations to be installed, divided between the North Sea and the Baltic Sea.
As the Baltic Sea limit of 750MW is expected to be reached by 2022, the authorised 700MW per year in the period 2023-2025 will almost certainly all be located in the North Sea.
"The acquisition of Atlantis I is for us a good start in the new year. Following the successful tenders in Denmark last year, where we were able to expand our offshore wind portfolio by 950MW, the acquisition of Atlantis I is also well placed on the German market," said Vattenfall's head of wind Gunnar Groebler
"In this context, we also welcome the switch to tenders under the new Renewable Energy Act (EEG). Now we can show that our know-how in terms of cost reduction for offshore wind can also be implemented in Germany for the benefit of electricity customers – which contributes to the acceptance of this effective form of green energy generation," he added.
In 2016, Vattenfall acquired the 632MW Global Tech II offshore wind project from the Erste Nordsee-Offshore-Holding development company.
At the time of the purchase, Groebler said the move sent "a signal that we strongly believe in the German market".